1. Field of the Invention
This invention relates to a surface acoustic wave filter and, more particularly, to the same that is formal with two surface acoustic wave resonators which comprise interdigital transducers and reflectors on a piezoelectric substrate and are transversely disposed with a minute spacing between them.
2. Description of the Prior Art
FIG. 12 is an illustration diagram showing one example of the conventional surface acoustic wave filters which are in the background of the present invention. The surface acoustic wave filter 1 comprises a piezoelectric substrate 2. On the piezo-electric substrate 2, two surface acoustic wave resonators 3 are formed. The surface acoustic wave resonator 3 includes an interdigital transducer 4 and two reflectors 5 formed on both sides of the interdigital transducer 4. These two surface acoustic wave resonators 3 are so formed that they are opposite to each other to become line-symmetry. From the two interdigital transducers 4, an input terminal 6 and an output terminal 7 are correspondingly extended, and an earth terminal 8 is also extended which connects the interdigital transducers 4 and reflectors 5.
Circuits equivalent to the surface acoustic wave filter 1 of FIG. 12 are shown in FIGS. 13 and 14. In the equivalent circuits of FIGS. 13 and 14, L.sub.1 =L.sub.s =L.sub.a, R.sub.1 =R.sub.s =R.sub.a, and 1/C.sub.m =1/C.sub.a -1/C.sub.s are predetermined. This surface acoustic wave filter 1 is formed as, for example, a bandpass type filter.
When such a surface acoustic wave filter is used, especially, in mobile communication equipment, steeper selectivity, that is, a good shape factor is required in many cases. In such a case, a method is generally used in which a plurality of the surface acoustic wave filters are cascade-connected to steepen the selectivity.
However, when a plurality of the surface acoustic wave filters are cascade-connected, the selectivity becomes steep and simultaneously an insertion loss falls large, and furthermore a passband width becomes narrower than it is required.